The present invention is directed to lasers and, more particularly, to linear lasers having a dual frequency coherent output.
In a number of applications it is desirable to measure various optical characteristics of an apparatus under study. In the ring laser art, for example, practitioners commonly measure the spectral linewidth of the resonant cavity forming the laser. That is, a measurement of the range of frequencies which can resonate or generate coherent radiation within the ring laser cavity The linewidths of other types of optical devices, such as linear laser cavities and etalons may also be of interest. Previously, the linewidths of resonant cavities have been determined according to conventional techniques such as the fast-finesse method and the ring-down method. Both of these procedures, however, suffer from inaccuracies resulting from several empirical measurements required by the processes.
A simpler and more accurate method of measuring cavity linewidth is disclosed in a co-pending patent application, Ser. No. 7/199,858, filed May 27, 1988, entitled "Multiple Input Source Technique For Deriving Cavity Line Width" which has also been assigned to Litton Systems, Inc. In accordance with the procedure disclosed in this co-pending application, the linewidth of a resonant cavity under study is determined by injecting a coherent beam containing two separate frequencies into the cavity under study and periodically varying the two frequencies while maintaining the separation between the frequencies a constant. An optical output from the cavity under study is then displayed as a function of intensity and frequency to provide a characteristic curve from which the cavity linewidth can be easily determined.
Conventional dual frequency lasers, such as Zeeman-effect lasers, are not easily adapted to the above-described dual frequency linewidth measurement technique since the separation of the two frequencies in the output beam of a Zeeman laser is normally dependent upon the actual value of the two output frequencies. As the two frequencies of the Zeeman laser are periodically varied or swept in accordance with the dual frequency measurement technique, the separation between the frequencies varies rather than remaining constant. Linear lasers providing a single frequency output are therefore typically employed in this technique and additional optical elements, such as a Pockels cell, are used to obtain a dual frequency beam of coherent radiation. Accordingly, there exists a need for a simpler method and apparatus to provide a dual frequency coherent output in which the two frequencies can be varied while maintaining the separation between the frequencies at a constant. The present invention fulfills this need.